1. FIELD OF THE INVENTION
The present invention relates to methods and apparatuses for controlling the shape, velocity direction, polarization and other characteristics of neutron beams.
2. DESCRIPTION OF PRIOR ART
Neutrons are important probes in material science because of the feature that they can interact with nuclei through strong interactions; their kinetic energy and wavelength are of the same order as atomic motion in matter and the scale of atomic structure, they have a magnetic moment and a strong penetrability, etc. Neutrons provide information of nuclei through nuclear interactions, while X-rays and photons provide information about atom structure through electromagnetic interactions. Therefore, neutron scattering experiments are necessary for the determination of the position and motion of nuclei regardless of the electron clouds of atoms.
The strength of neutron-nuclear interactions are irregular with respect to the atomic number of elements and dependence on the mass number of isotopes, while the strength of electromagnetic interactions have a monotonous dependence only on the atomic number. This feature is applied to distinguish elements which have similar electromagnetic scattering strengths and isotopes of an atomic number. It is also applicable for determining the position and motion of light elements such as the study of hydrogen atoms in organic materials.
The neutron magnetic dipole moment originates from its 1/2 spin and is suitable for the study of the magnetic structure of matter. The strong penetrability can be applied to investigate the macroscopic structure of bulk samples such as industrial products, which are difficult to investigate using charged particles and X-rays.
The efficient use of neutron beams is very important since neutron beams are available at limited facilities equipped with nuclear reactors, accelerators and strong radioactive sources. Improvement of neutron beam transport from a neutron source to a neutron spectrometer is strongly desired since the improvement of neutron source intensity is limited by both cost and radiation control technique. The improvement not only reduces measurement time but also enables us to carry out in situ measurements of transient phenomena and to study the structure of new materials for which large scale single crystals are not available. It also reduces the risks in radiation safeties.
Neutron guides are commonly used in neutron transport. Neutron beams can be bent according to their reflection on the interface of matters with a sufficiently small incident angle. Neutron guides are vacuum tubes that have an inner surface that is coated with a neutron reflector such as nickel and are pumped to a vacuum to minimize the loss of neutrons through scattering by air. Neutrons incident to the guide with an angle smaller than the critical angle of the neutron reflector material are reflected on the inner surface and transported downstream.
FIG. 17A is an illustration of the concept of neutron scattering and FIG. 17B is an enlarged view around the sample. Neutrons are emitted in all directions from the neutron source 100: nuclear reactors or radioactive sources or nuclear target bombarded by charged particles. A part of the neutrons that are generated are then transported by the neutron guide 101 and incident to the sample 102. A neutron detector, such as a proportional counter 103, measures the intensity of neutrons scattered at an angle of .theta.. The angular distribution of the scattered neutrons is analyzed to extract information related to the atomic structure of the sample. The typical aperture of the neutron guide 101 is about 5 cm and the typical size of the sample 102 is 1-2 cm or larger.
One of the existing devices that increase beam density are neutron capillary tubes. Neutron capillary tubes are bundled tubes 110 which have thin channels with diameters of about 10 .mu.m as shown in FIG. 18. Incident neutrons are transported by reflection on the inner surface of the channels. Neutron beam density is improved by adjusting the curvature of each tube 101 so that the exiting neutrons are focused on a small area 113.
The beam divergence of the incident beam should be sufficiently small for good resolution in determining scattering angles since scattering angles cannot be determined precisely if the incident beam is divergent. A is common method to reduce beam divergence is by neutron diffraction. However, beam intensity is attenuated to much upon diffraction.
Dense and thin neutron beams are strongly desired in the analysis of new materials since large samples of 1-2 cm cannot easily be prepared. Small divergence of incident beams are also required to determine the atomic structure of a sample.
Neutron guides can transport neutrons efficiently but cannot focus nor reduce beam divergence. Neutron beams 104 emitted at the exit of neutron guide 101 are divergent. Neutrons with the scattering angles of .theta..sub.1, .theta..sub.2, . . . are detected by the same detector 103 as shown in FIG. 17B. This causes a non-negligible error in determining the scattering angles. Beam collimators are placed upstream from the sample to reduce the error which suppresses the efficiency the neutron use.
Neutron capillary tubes increase neutron beam density.
However, the efficiency of the neutron use is suppressed, as shown in FIG. 18, because only the neutrons transported through the thin channels are focused downstream and neutrons 112 that pass between tubes 110 are not used. In addition, since the tubes 110 are curved to bring the neutrons into convergence, beam divergence is enlarged at the focal point; this is not suitable for good angular resolution.
Neutron diffraction by a single crystal can suppress neutron beam divergence. However, the beam intensity is attenuated to much.
Existing methods related to neutron beam control are not appropriate for obtaining a thin and dense beam.